Method of isolating surface tension and yield stress in viscosity measurements

ABSTRACT

A method for isolating the effects of surface tension and/or yield stress of a fluid that is flowing in a U-shaped tube wherein one or both legs of the U-shaped tube is monitored over time for the changing height of the respective fluid columns therein. A portion of the U-shaped tube comprises a flow restrictor, e.g., a capillary tube, of known dimensions. Monitoring one or both of the moving fluid columns over time permits the determination of the viscosity of the fluid flowing therein over a range of shear rates from the difference in fluid column heights. However, it is necessary to isolate the effects of surface tension and/or yield stress to obtain an accurate viscosity determination. The method provides one manner in which the surface tension effect can be subtracted from the difference in fluid column heights and then any yield stress effect can then be determined. Alternatively, the method also provides a process by which both the surface tension effect and yield stress effect can be determined simultaneously.

BACKGROUND OF THE INVENTION

This application is a Continuation-In-Part of application Ser. No. 09/573,267, filed May 18, 2000, now U.S. Pat. No. 6,402,703 which is a Continuation-in-Part of application Ser. No. 09/439,795, filed Nov. 12, 1999, now U.S. Pat. No. 6,322,524 both of which are entitled DUAL RISER/SINGLE CAPILLARY VISCOMETER, which is turn is a Continuation-In-Part of application Ser. No. 08/919,906 filed Aug. 28,1997 entitled VISCOSITY MEASURING APPARATUS AND METHOD OF USE, now U.S. Pat. No.6,019,735, all of which are assigned to the same Assignee, namely Visco Technologies, Inc. as the present invention and all of whose entire disclosures are incorporated by reference herein.

FIELD OF INVENTION

This invention relates generally to the field of measuring the viscosity of liquids, and more particularly, to a method of isolating the surface tension and yield stress effects when determining the viscosity of a liquid using a U-shaped scanning capillary tube viscometer.

BACKGROUND OF THE INVENTION

In a scanning capillary tube viscometer, a U-shaped tube is used where one portion of the U-shaped tube is formed by a flow restrictor, e.g., capillary tube. One leg of the U-shaped tube supports a falling column of fluid and the other leg supports a rising column of fluid Furthermore, movement of either one or both of these columns is monitored, hence the term “scanning.” See FIG. 1. It should be understood that the term “scanning,” as used in this Specification, also includes the detection of the change in mass (e.g., weight) in each of the columns. Thus, all manners of detecting the change in the column mass, volume, height, etc. is covered by the term “scanning.”

In order to measure liquid viscosity using a U-shaped scanning capillary tube viscometer, the pressure drop across the capillary tube has to be precisely estimated from the height difference between the two fluid columns in the respective legs of the U-shaped tube. However, under normal circumstances, the height difference, Δh(t), contains the effects of surface tension and yield stress. Therefore, the contributions of the surface tension (Δh_(st)) and yield stress (τ_(y)) to Δh(t), have to be taken into account, or isolated. Here, Δh(t) is equal to h₁(t)−h₂(t).

The magnitude of the surface tension of a liquid in a tube differs greater depending on the condition of the tube wall. Normally, the surface tension reported in college textbooks are measured from a perfectly wet tube. However, in reality, the falling column has a perfectly wet surface while the rising column has a perfectly dry surface. When the tube is completely dry, the value of the surface tension from the same tube can be substantially different from that measured from a perfectly wet tube. Hence, there is a pronounced effects of the surface tension on the overall height difference between the two columns. The height difference caused by the surface tension can be significantly greater than the experimental resolution required for the accuracy of viscosity measurement. For example, the difference between surface tensions of two columns in the U-shaped tube can produce the height difference, Δh_(st), of 3.5 mm where the height difference, Δh (t), must be measured as accurately as 0.1 mm. Thus, it is extremely important to isolate the effect of the surface tension from the viscosity measurement.

Similarly, the effect of yield stress, τ_(y), must be isolated from the viscosity measurement.

Thus, there remains a need for accounting for, or isolating, the surface tension and yield stress in viscosity measurements when using a scanning capillary tube viscometer.

SUMMARY OF THE INVENTION

A method for isolating the effect of surface tension on a fluid that is flowing in a U-shaped tube having a flow restrictor (e.g., a capillary tube) forming a portion of said U-shaped tube. The fluid forms a falling column of fluid, having a first height that changes with time, in a first leg of the U-shaped tube and a rising column of fluid, having a second height that changes with time, in a second leg of said U-shaped tube. The method comprises the steps of: (a) detecting the difference between the first and second heights over time; and (b) subtracting a term representing surface tension from the difference.

A method of isolating the effect of surface tension on a fluid and the effect of yield stress of a fluid that is flowing in a U-shaped tube having a flow restrictor forming a portion of said U-shaped tube. The fluid forms a falling column of fluid, having a first height that changes with time, in a first leg of said U-shaped tube and a rising column of fluid, having a second height that changes with time, in a second leg of said U-shaped tube. The method comprises the steps of: (a) detecting the difference between the first and second heights over time for generating falling column data and rising column data; (b) curve fitting an equation using the falling column data and the rising column data to determine: (1) a term representing surface tension; and (2) a term representing the yield stress.

DESCRIPTION OF THE DRAWINGS

The invention of this present application will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a functional diagram of a test fluid flowing in a U-shaped tube having a flow restrictor therein and with column level detectors and a single point detector monitoring the movement of the fluid; and

FIG. 2 is a graphical representation of the height of the respective columns of fluid over time in the two legs of the U-shaped tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring now in detail to the various figures of the drawing wherein like reference characters refer to like parts, there is shown at 20 a U-tube structure comprising a first leg (e.g., a first riser tube) R1, a flow restrictor 22 (e.g., a capillary tube) and a second leg (e.g., second riser tube) R2. It should be understood that the preferred embodiment of the U-tube structure has the flow restrictor forming the horizontal portion at the bottom of the U-tube that connects the two legs together. However, it is within the broadest scope of this invention to include the positioning of the flow restrictor in either one of the legs themselves.

The apparatus 20 uses column level detectors 54/56 for detecting the movement (e.g., the heights, h) of the columns of fluid in the legs of the U-tube: a falling column of fluid 82 and a rising column of fluid 84, as indicated by the respective arrows 83 and 85. The details of such types of detectors are disclosed in application Ser. No. 09/439,795, now U.S. Pat. No. 6,322,524 filed on Nov. 12, 1999 and application Ser. No. 09/573,267, now U.S. Pat. No. 6,403,703 filed on May 18, 2000, now U.S. Pat. No. 6,403,703 both of which are entitled DUAL RISER/SINGLE CAPILLARY VISCOMETER, both of which are assigned to the same Assignee of the present invention, namely Visco Technologies, Inc. and both of whose entire disclosures are incorporated by reference herein Thus, the details of these detectors are not repeated here. Furthermore, although not shown in FIG. 1, but disclosed in these other applications, the column level detectors 54/56 communicate with a computer for processing the data collected by these detectors.

It should be understood that it is within the broadest scope of this invention to include the monitoring of only one of the columns of fluid 82 or 84 while obtaining a single point from the other one of the columns of fluid 82 or 84 using a single point detector 954 (which also communicates with the computer mentioned previously). In particular, as shown in FIG. 2, since the rising and falling columns 82/84 exhibit a symmetry about a horizontal axis, it is possible to monitor only one of the columns of fluid while obtaining a single data point from the other column. For example, as shown in FIG. 1, the column level detector 56 can be used to monitor the rising column 84 while the single point detector 954 can be used to detect any one point of the falling column 82 in R1.

In the preferred embodiment, the U-tube structure 20 is in an upright position; the test fluid is entered into the top of one of the legs (e.g., R1) while the top of the other leg (e.g. R2) is exposed to atmospheric pressure. Using this configuration, the test fluid is subjected to a decreasing pressure differential that moves the test fluid through a plurality of shear rates (i.e., from a high shear rate at the beginning of the test run to a low shear rate at the end of the test run), which is especially important in determining the viscosity of non-Newtonian fluids, as set forth in application Ser. Nos. 09/439,795 and 09/573,267. However, it should be understood that it is within the broadest scope of this invention to include any other configurations where the test fluid can be subjected to a decreasing pressure differential in order to move the test fluid through a plurality of shear rates.

As disclosed in those patent applications, the height vs. time data that was generated is shown in FIG. 2, where as time goes to infinity a constant separation between the column heights, known as Δh_(∞) can be attributed to surface tension Δh_(st) and/or yield stress τ_(y). The present application provides a method for determining the individual effects of these two parameters. In particular, the present application discloses a mathematical method to isolate both the effects of surface tension and yield stress of a test fluid from the pressure drop created by the height difference between the two columns of test fluid in respective legs of the U-shaped tube.

The method begins with the conservation of energy equation which can be written in terms of pressure as follows: $\begin{matrix} {{P_{1} + {\frac{1}{2}\rho \quad V_{1}^{2}} + {\rho \quad {{gh}_{1}(t)}}} = {P_{2} + {\frac{1}{2}\rho \quad V_{2}^{2}} + {\rho \quad {{gh}_{2}(t)}} + {\Delta \quad P_{c}} + {\rho \quad g\quad \Delta \quad h_{st}}}} & (1) \end{matrix}$

where

P₁ and P₂: hydro-static pressures at fluid levels at the two columns 82/84 in the U-shaped tube;

ρ: density of fluid;

g: gravitational acceleration;

V₁ and V₂: flow velocities of the two columns of fluid 82/84 in the U-shaped tube; h₁(t) and h₂(t): heights of the two columns of fluid 82/84 in the U-shaped tube;

ΔP_(c)(t): pressure drop across the capillary tube 22;

Δh_(st): additional height difference due to surface tension;

Since P₁ and P₂=P_(atm) and |V₁ |=|V₂|, equation (1) can be reduced to the following:

ΔP _(c)(t)=ρg[h ₁(t)−h ₂(t)−Δh_(st)]  (2)

This last equation demonstrates that the effect of the surface tension is isolated by subtracting Δh_(st) from the height difference, h₁(t)−h₂(t), between the two columns of fluid 82/84 in the U-shaped tube; the yield stress τ_(y) will be addressed later. Thus, the pressure drop across a capillary tube 22 can be determined as shown in Equation (2). Later, Δh_(st) is determined through curve fitting of the experimental data of h₁(t) and h₂(t).

It should be understood that Equation 2 is valid regardless of the curve-fitting model selected, i.e., power-law, Casson, or Herschel-Bulkley model. Furthermore, as used throughout this Specification, the phrase “curve fitting” encompasses all manners of fitting the data to a curve and/or equation, including the use of “solvers”, e.g., Microsoft's Excel Solver that can solve for a plurality of unknowns in a single equation from data that is provided.

When a fluid exhibits a yield stress (τ_(y)), the height difference between the two fluid columns of the U-shaped tube is greatly affected by the yield stress particularly at low shear operation. Accordingly, the effect of the yield stress also must be taken into account (i.e., isolated) in order to accurately estimate the pressure drop across the capillary tube. In order to handle the yield stress term, one needs to start with a constitutive equation such as the Casson or Herschel-Bulkley model. These constitutive models have a term representing the yield stress as set forth below:

Casson model:

{square root over (τ)}={square root over (τ_(y))}+{square root over (k)}{square root over ({dot over (γ)})}, when τ>τ_(y)

{dot over (γ)}=0, τ<τ_(y)

where τ is shear stress, {dot over (γ)} is shear rate, τ_(y) is the yield stress, and k is a constant.

Herschel-Bulkley model:

τ=τ_(y) +k{dot over (γ)} ^(n) when τ≦τ_(y)

{dot over (γ)}=0, when τ<τ_(y)

where k and n are model constants.

It should be noted that the power-law model does not have the yield stress term. Thus, it does not have the capability of handling the effect of yield stress.

For both the Casson and Herschel-Bulkley models, the pressure drop across the capillary tube 22, ΔP_(c)(t), can be expressed as follows:

ΔP _(c)(t)=ρg [h ₁(t)−h ₂(t)−Δh _(st)]  (3)

As time goes to infinity, this equation can be written as

ΔP _(c)(∞)=ρg[h ₁(∞)−Δh _(st)]  (4)

where ΔP_(c)(∞) represents the pressure drop across the capillary tube 22 as time goes to infinity, which can be attributed to the yield stress, τ_(y), of the test fluid. There is a relationship between the yield stress, τ_(y), and ΔP_(c)(∞), which can be written as: $\begin{matrix} {\tau_{y} = \frac{\Delta \quad {{P_{c}(\infty)} \cdot R}}{2L}} & (5) \end{matrix}$

where R and L are the radius and the length of the capillary tube 22, respectively.

To obtain the Δh_(st) and τ_(y), two alternative approaches can be used. In the first approach, these two parameters are obtained sequentially, i.e., Equation 4 is curve-fitted using the experimental data of h₁(t) and h₂(t) data and solved for Δh_(st); then the determined value for Δh_(st) is plugged into Equation 5 and solved for τ_(y).

Alternatively, in the second approach, both Δh_(st) and the yield stress can be determined directly from the curve fitting of the experimental data of h₁(t) and h₂(t), as set forth below. As mentioned previously, in order to handle the yield stress τ_(y), a constitutive equation is required, which has a term representing the yield stress. Examples of such a constitutive equation include Casson and Herschel-Bulkley models, although these are only by way of example and not limitation. In particular, the procedure for curve-fitting the column height data with either a Casson model or a Herschel-Bulkley model is as follows:

From the falling column of fluid 82, h₁(t), and the rising column of fluid 84, h₂(t), one can obtain the flow velocities of the two columns by taking the derivative of each height, i.e., $\begin{matrix} {V_{1} = {\frac{{h_{1}(t)}}{t}\quad {and}}} & (6) \\ {V_{2} = \frac{{h_{2}(t)}}{t}} & (7) \end{matrix}$

Since the flows in the two columns of the U-shaped tube move in the opposite directions, one can determine the average flow velocity at the riser tube, {overscore (V)}_(r), by the following equation: $\begin{matrix} {V_{r} = {{\frac{1}{2}\left( {V_{1} - V_{2}} \right)} = {\frac{1}{2}\left( {\frac{{h_{1}(t)}}{t} - \frac{{h_{2}(t)}}{t}} \right)}}} & (8) \end{matrix}$

Since the scanning capillary tube viscometer collects h₁(t) and h₂(t) data over time, one can digitize the h₁(t) and h₂(t) data in the following manner to obtain the average flow velocity at the riser tube: $\begin{matrix} {\frac{\left( {{h_{1}(t)} - {h_{2}(t)}} \right)}{t} = \frac{\left( {{h_{1}\left( {t + {\Delta \quad t}} \right)} - {h_{2}\left( {t + {\Delta \quad t}} \right)}} \right) - \left( {{h_{1}\left( {t - {\Delta \quad t}} \right)} - {h_{2}\left( {t - {\Delta \quad t}} \right)}} \right)}{2\quad \Delta \quad t}} & (9) \end{matrix}$

In order to start a curve-fitting procedure, a mathematical description of V_(r) is required for each model:

1. Casson model

{square root over (τ)}={square root over (τ_(y))}+{square root over (k)}{square root over ({dot over (γ)})} when τ≧τ_(y)

{dot over (γ)}=0, when τ<τ_(y)

where τ is shear stress, {dot over (γ)} is shear rate, τ_(y) is the yield stress, and k is a constant.

Wall shear stress (τ_(w)) and yield stress (τ_(y)) can be defined as follows: ${\tau_{w} = \frac{\Delta \quad {{P_{c}(t)} \cdot R}}{2L}},{\tau_{y} = {{\frac{\Delta \quad {{P_{c}(t)} \cdot {r_{y}(t)}}}{2L}\left( {{evaluated}\quad {as}\quad {time}\quad {goes}\quad {{to}\infty}} \right)} = \frac{\Delta \quad {{P_{c}(\infty)} \cdot R}}{2L}}},$

where R and L are the radius and length of a capillary tube, respectively, and r_(y)(t) is the radial distance, where shear stress (τ) is bigger than yield stress (τ_(y)).

Using the above equations, the velocity profile at the capillary tube 22 can be calculated as follows: ${V\left( {t,r} \right)} = {{{{\frac{1}{4k} \cdot {\frac{\Delta \quad {P_{c}(t)}}{L}\left\lbrack {R^{2} - r^{2} - {\frac{8}{3}{r_{y}^{\frac{1}{2}}(t)}\left( {R^{\frac{3}{2}} - r^{\frac{3}{2}}} \right)} + {2{r_{y}(t)}\left( {R - r} \right)}} \right\rbrack}}\quad {for}\quad {r_{y}(t)}} \leq r \leq {{RV}(t)}} = {{{\frac{1}{4k} \cdot \frac{\Delta \quad {P_{c}(t)}}{L}}\left( {\sqrt{R} - \sqrt{r_{y}(t)}} \right)^{3}\left( {\sqrt{R} + {\frac{1}{3}\sqrt{r_{y}(t)}}} \right)\quad {for}\quad {r_{y}(t)}} \geq r}}$

According to the definition of shear rate, {dot over (})}, the expression of the shear rate is defined as: $\overset{.}{\gamma} = {{- \frac{V}{r}} = {\frac{1}{k}{\left( {\sqrt{\frac{\Delta \quad {{P_{c}(t)} \cdot r}}{2L}} - \sqrt{\frac{\Delta \quad {{P_{c}(t)} \cdot {r_{y}(t)}}}{2L}}} \right).}}}$

In order to calculate the average flow velocity at either riser tube ({overscore (V)}_(r)), the flow rate at a capillary tube 22 needs to be determined first. The flow rate at the capillary tube 22 can be obtained by integrating the velocity profile over the cross-section of the capillary tube 22 as: $\begin{matrix} {{Q(t)} = \quad {2\pi {\int_{0}^{R}{{Vr}\quad {r}}}}} \\ {= \quad {\frac{\pi \quad R^{4}}{8k}\left\lbrack {\frac{\Delta \quad {P_{c}(t)}}{L} - {{\frac{16}{7} \cdot \left( \frac{2\quad \tau_{y}}{R} \right)^{\frac{1}{2}}}\left( \frac{\Delta \quad {P_{c}(t)}}{L} \right)^{\frac{1}{2}}} +} \right.}} \\ {\left. \quad {{\frac{4}{3}\left( \frac{2\quad \tau_{y}}{R} \right)} - {{\frac{1}{21} \cdot \left( \frac{2\quad \tau_{y}}{R} \right)^{4}}\left( \frac{\Delta \quad {P_{c}(t)}}{L} \right)^{- 3}}} \right\rbrack.} \end{matrix}$

Since Q(t)=πR_(r) ²{overscore (V)}_(r), the mean flow velocity at the riser tube can be calculated as follows: $\begin{matrix} {{\overset{\_}{V}}_{r} = \quad {{\frac{R^{4}}{8{kR}_{r}^{2}}\left\lbrack \frac{\Delta \quad {P_{c}(t)}}{L} \right.} - {{\frac{16}{7} \cdot \left( \frac{2\quad \tau_{y}}{R} \right)^{\frac{1}{2}}}\left( \frac{\Delta \quad {P_{c}(t)}}{L} \right)^{\frac{1}{2}}} +}} \\ \left. \quad {{\frac{4}{3} \cdot \left( \frac{2\quad \tau_{y}}{R} \right)} - {{\frac{1}{21} \cdot \left( \frac{2\tau_{y}}{R} \right)^{4}}\left( \frac{\Delta \quad {P_{c}(t)}}{L} \right)^{- 3}}} \right\rbrack \end{matrix}$

where R_(r) is the radius of either riser tube. Thus, a mathematical description for {overscore (V)}_(r) for Casson model case is obtained.

Using a curve-fitting model (e.g., the Microsoft Excel Solver), the unknown variables can be determined. For the Casson model, there are three unknown variables, which are the model constant, k, the contribution of the surface tension Δh_(st) and the yield stress τ_(y). It should be noted that the unknown variables are constants, which will be determined from the curve-fitting of experimental data of ΔP_(c) (t). It should also be noted that ΔP_(c) (t) essentially comes from h₁(t) and h₂(t)

2. Herschel-Bulkley model

τ=τ_(y) +k{dot over (γ)} ^(n) when τ≧τ_(y)

{dot over (γ)}=0, when τ<τ_(y)

where k and n are both model constants.

In a similar method as for the case of Casson model, the velocity profile at the capillary tube 22 can be derived for the Herschel-Bulkley model as follows: $\begin{matrix} {{V\left( {t,r} \right)} = \quad {\left( \frac{\Delta \quad {P_{c}(t)}}{2{kL}} \right)^{\frac{1}{n}}{\left( \frac{n}{n + 1} \right)\left\lbrack {\left( {R - {r_{y}(t)}} \right)^{\frac{n + 1}{n}} - \left( {r - {r_{y}(t)}} \right)^{\frac{n + 1}{n}}} \right\rbrack}}} & {\quad {{{for}\quad {r_{y}(t)}} \leq r \leq R}} \\ {{V(t)} = \quad {\left( \frac{\Delta \quad {P_{c}(t)}}{2{kL}} \right)^{\frac{1}{2}}\left( \frac{n}{n + 1} \right)\left( {R - {r_{y}(t)}} \right)^{\frac{n + 1}{n}}}} & {\quad {{{for}\quad {r_{y}(t)}} \geq r}} \end{matrix}$

The flow rate at capillary tube 22 can be obtained by integrating the velocity profile over the cross-section of the capillary tube 22 as: $\begin{matrix} {{Q(t)} = \quad {2\pi {\int_{0}^{R}{{Vr}\quad {r}}}}} \\ {= \quad {{{\pi \left( \frac{\Delta \quad {P_{c}(t)}}{2{kL}} \right)}^{\frac{1}{n}}\left( \frac{n}{n + 1} \right)R^{\frac{{3n} + 1}{n}}\left( {C_{y}^{2}\left( {1 - C_{y}} \right)}^{\frac{n + 1}{n}} \right.} +}} \\ {\quad {{\left( {1 + C_{y}} \right)\left( {1 - C_{y}} \right)^{\frac{{2n} + 1}{n}}} - {\left( \frac{2n}{{2n} + 1} \right){C_{y}\left( {1 - C_{y}} \right)}^{\frac{{2n} + 1}{n}}} -}} \\ {\quad \left. {\left( \frac{2n}{{3n} + 1} \right)\left( {1 - C_{y}} \right)^{\frac{{3n} + 1}{n}}} \right)} \end{matrix}$

where, ${{C_{y}(t)} = \frac{r_{y}(t)}{R}},$

which is used for convenience.

Since Q(t)=πR₂ ^(r){overscore (V)}_(r), ${\overset{\_}{V}}_{r} = {\left( \frac{1}{R_{r}^{2}} \right)\left( \frac{\Delta \quad {P_{c}(t)}}{2{kL}} \right)^{\frac{1}{n}}\left( \frac{n}{n + 1} \right){R^{\frac{{3n} + 1}{n}}\left( {{C_{y}^{2}\left( {1 - C_{y}} \right)}^{\frac{n + 1}{n}} + {\left( {1 + C_{y}} \right)\left( {1 - C_{y}} \right)^{\frac{{2n} + 1}{n}}} - {\left( \frac{2n}{{2n} + 1} \right){C_{y}\left( {1 - C_{y}} \right)}^{\frac{{2n} + 1}{n}}} - {\left( \frac{2n}{{3n} + 1} \right)\left( {1 - C_{y}} \right)^{\frac{{3n} + 1}{n}}}} \right)}}$

where R_(r) is the radius of either riser tube. Thus a mathematical description for {overscore (V)}_(r) for the Herschel-Bulkley model case is obtained. Using a curve-fitting model (e.g., Microsoft Excel Solver), the unknown variables can be determined. For the Herschel-Bulkley model, there are four unknown variables, which are the two model constants, n and k, the contribution of the surface tension, Δh_(st), and yield stress τ_(y). Again, it should be noted that the unknown variables are constants, which will be determined from the curve-fitting of experimental data of ΔP_(c)(t). It should also be noted that ΔP_(c)(t) essentially comes from h₁(t) and h₂(t).

It should be understood that the above disclosed methodology, including the equations, curve fitting, etc., can be implemented on the computer that communicates with the column level detectors 54/56 and/or the single point detector 954.

It should also be understood that the actual position of the flow restrictor 22 is not limited to the horizontal portion of the U-shaped tube; the flow restrictor 22 could form a portion of either leg of the U-shaped tube as shown in application Ser. Nos. 09/439,795 and 09/573,267.

Without further elaboration, the foregoing will so fully illustrate our invention that others may, by applying current or future knowledge, readily adopt the same for use under various conditions of service. 

We claim:
 1. A method for isolating the effect of surface tension on a fluid that is flowing in a U-shaped tube having a flow restrictor forming a portion of said U-shaped tube, said fluid forming a falling column of fluid, having a first height that changes with time, in a first leg of said U-shaped tube and a rising column of fluid, having a second height that changes with time, in a second leg of said U-shaped tube, said method comprising the steps of: (a) detecting the difference between said first and second heights over time; and (b) subtracting a term representing surface tension from said difference.
 2. The method of claim 1 wherein said step of detecting said difference between said first and second heights comprises monitoring the movement over time of at least one of said columns of fluid, while detecting a single data point of the other one of said columns of fluid for generating rising column data and falling column data.
 3. The method of claim 2 wherein said at least one of said columns of fluid is said rising column of fluid and said other one of said columns of fluid is said falling column of fluid.
 4. The method of claim 1 wherein said step of detecting the difference between said first and second heights comprises monitoring the movement of both of said columns of fluid over time for generating rising column data and falling column data.
 5. The method of claim 4 wherein said step of subtracting a term representing surface tension comprises: (a) selecting a first equation that represents a pressure drop across said flow restrictor in terms of said first and second heights; and (b) curve fitting said first equation using said rising column data and said falling column data to determine said term representing surface tension.
 6. The method of claim 5 wherein said flow restrictor is a capillary tube of known dimensions and wherein said first equation comprises: ΔP _(c)(t)=ρg [h ₁(t)−h ₂(t)−Δh _(st)], where, ΔP_(c)(t) is said pressure drop across said capillary tube; ρ is the density of said fluid; g is the gravitational acceleration; h₁(t) is said first height over time; h₂(t) is said second height over time; and Δh_(st) is said term representing surface tension.
 7. The method of claim 6 further comprising the step of determining the yield stress, τ_(y), of the fluid, said step of determining the yield stress comprises solving a second equation: ${\tau_{y} = \frac{\Delta \quad {{P_{c}(\infty)} \cdot R}}{2L}},$

where, ΔP_(c),(∞) is given by: ρg[h₁(∞)−h₂(∞)−Δh_(st)]; h₁(∞) is said first height after a long period of time; h₂(∞) is said second height after a long period of time; R is the radius of said capillary tube; and L is the length of said capillary tube.
 8. The method of claim 2 wherein said step of subtracting a term representing surface tension comprises: (a) selecting a first equation that represents a pressure drop across said flow restrictor in terms of said first and second heights; and (b) curve fitting said first equation using said rising column data and said falling column data to determine said term representing surface tension.
 9. The method of claim 8 wherein said flow restrictor is a capillary tube of known dimensions and wherein said first equation comprises: ΔP _(c)(t)=ρg[h ₁(t)−h ₂(t)−Δh _(st],) where, ΔP_(c)(t) is said pressure drop across said capillary tube; ρ is the density of said fluid; g is the gravitational acceleration; h₁(t) is said first height over time; h₂(t) is said second height over time; and Δh_(st) is said term representing surface tension.
 10. The method of claim 9 further comprising the step of determining the yield stress, τ_(y), of the fluid, said step of determining the yield stress comprises solving a second equation: ${\tau_{y} = \frac{\Delta \quad {{P_{c}(\infty)} \cdot R}}{2L}},$

where, ΔP_(c)(∞) is given by: ρg[h₁(∞)−h₂(∞)−Δh_(st)]; h₁(∞) is said first height after a long period of time; h₂(∞) is said second height after a long period of time; R is the radius of said capillary tube; and L is the length of said capillary tube.
 11. A method of isolating the effect of surface tension on a fluid and the effect of yield stress of a fluid that is flowing in a U-shaped tube having a flow restrictor forming a portion of said U-shaped tube, said fluid forming a falling column of fluid, having a first height that changes with time, in a first leg of said U-shaped tube and a rising column of fluid, having a second height that changes with time, in a second leg of said U-shaped tube, said method comprising the steps of: (a) detecting the difference between said first and second heights over time for generating falling column data and rising column data; (b) curve fitting an equation using said falling column data and said rising column data to determine: (1) a term representing surface tension; and (2) a term representing said yield stress.
 12. The method of claim 11 wherein said equation comprises a representation of the average velocity of either said falling column or said rising column of fluid.
 13. The method of claim 12 wherein said equation utilizes a Casson model that defines fluid shear stress, τ, in terms of yield stress, τ_(y), and shear rate {dot over (γ)}, as follows: {square root over (τ)}={square root over (τ_(y))}+{square root over (k)}{square root over ({dot over (γ)})} when τ≧τ_(y) and {dot over (γ)}=0 when τ<τ_(y); and where k is a model constant.
 14. The method of claim 13 wherein said flow restrictor is a capillary tube of known dimensions and wherein said equation representing said velocity of either said falling column or said rising column is defined as: ${V = {\frac{R^{4}}{8{kR}_{r}^{2}}\left\lbrack {\frac{\Delta \quad P_{c{(t)}}}{L} - {{\frac{16}{7} \cdot \left( \frac{2\tau_{y}}{R} \right)^{\frac{1}{2}}}\left( \frac{\Delta \quad {P_{c}(t)}}{L} \right)^{\frac{1}{2}}} + {\frac{4}{3} \cdot \left( \frac{2\tau_{y}}{R} \right)} - {{\frac{1}{21} \cdot \left( \frac{2\tau_{y}}{R} \right)^{4}}\left( \frac{\Delta \quad {P_{c}(t)}}{L} \right)^{- 3}}} \right\rbrack}},$

where: R is the radius of said capillary tube; L is the length of said capillary tube; R_(r) is the radius of said falling column of fluid or said rising column of fluid; and ΔP_(c)(t)=ρg[h₁(t)−h₂(t)−Δh_(st)]; where: ΔP_(c)(t) is a pressure drop across said capillary tube; ρ is the density of said fluid; g is the gravitational acceleration; h₁(t) is said first height over time; h₂(t) is said second height over time; and Δh_(st) is said term representing surface tension.
 15. The method of claim 12 wherein said equation utilizes a Herschel-Bulkley model that defines fluid shear stress, τ, in terms of yield stress, τ_(y), and shear rate {dot over (γ)}, as follows: τ=τ_(y) +k{dot over (γ)} ^(n) when τ≧τ_(y) and {dot over (γ)}=0 when τ<τ_(y); and where k and n are model constants.
 16. The method of claim 15 wherein said flow restrictor is a capillary tube of known dimensions and wherein said equation representing said velocity (V) of either said falling column or said rising column is defined as: ${\left. {V = {{\left( \frac{1}{R_{r}^{2}} \right)\left( \frac{\Delta \quad {P_{c}(t)}}{2{kL}} \right)^{\frac{1}{n}}\left( \frac{n}{n + 1} \right){R^{\frac{{3n} + 1}{n}}\left( {{C_{y}^{2}\left( {1 - C_{y}} \right)}^{\frac{n + 1}{n}} +} \right.}\left( {1 + C_{y}} \right)\left( {1 - C_{y}} \right)^{\frac{{2n} + 1}{n}}} - {\left( \frac{2n}{{2n} + 1} \right){C_{y}\left( {1 - C_{y}} \right)}^{\frac{{2n} + 1}{n}}} - {\left( \frac{2n}{{3n} + 1} \right)\left( {1 - C_{y}} \right)^{\frac{{3n} + 1}{n}}}}} \right),\quad {{where}\text{:}}}\quad$ ${{C_{y}(t)} = \frac{r_{y}(t)}{R}};$

R is the radius of said capillary tube; r_(y)(t) is the radial distance and where τ>τ_(y); L is the length of said capillary tube; R_(r) is the radius of said falling column of fluid or said rising column of fluid; and ΔP_(c)(t)=ρg[h₁(t)−h₂(t)−Δh_(st)]; where: ΔP_(c)(t) is a pressure drop across said capillary tube; ρ is the density of said fluid; g is the gravitational acceleration; h₁(t) is said first height over time; h₂(t) is said second height over time; and Δh_(st) is said term representing surface tension. 